Magnetic tape media is often used for storage and retrieval of data, and comes in many widths and lengths. Magnetic tape media remains an economical medium for storing large amounts of data. For example, magnetic tape cartridges, or large spools of magnetic tape are used to back up large amounts of data for large computing centers. Magnetic tape cartridges also find application in the backup of data stored on smaller computers such as desktop or laptop computers.
During the lifespan of magnetic tape, it may be necessary or desirable to erase the tape. For example, an erasing process is typically implemented during the fabrication of magnetic tape media. Typically, magnetic tape is erased after being coated, cut and spooled, and then servo tracks are written on the magnetic tape after it has been erased. At this stage, there typically is no data stored on the tape. Nevertheless, it is desirable to perform erasure prior to servo writing to ensure that the servo patterns can be properly written.
In addition, magnetic tape may also be erased later in the lifespan of the magnetic tape. For example, the tape may be erased after being recorded, thus putting the tape into a condition to be re-recorded. In this document, the term “erasure” refers to a process or an apparatus that returns the magnetic tape or a portion thereof to a substantially demagnetized condition, i.e., a condition in which the fields of the individual magnetic particles on the tape substantially cancel out. Often the demagnetized condition is one in which the individual magnetic particles have substantially no ordered magnetization pattern.
Erasure is typically accomplished by subjecting the magnetic tape to a magnetic field of sufficient magnitude to saturate the magnetic particles on the tape. The polarity of the applied field is then reversed and the magnitude of the applied field is reduced by some small amount. The polarity of the applied field is then reversed again and the magnitude is again reduced. The process of reducing the magnitude of the applied field and reversing the polarity of the applied field continues until the magnitude reaches zero, or some finite value which is determined to be sufficiently small as to have no further effect.
One common method of erasing involves subjecting an entire reel of tape to a field which is generated by an alternating electric current. The alternating current produces the reversal of magnetic field, and the decay in field strength is provided either by reducing the magnitude of the applied current or by physically removing the media from the erasing field.
Another method of erasing implements a specially designed recording head driven with alternating current. As the tape passes by the head, it is erased by the alternating field generated by the recording head. In order to leave the media in a demagnetized state, each point of the media is subjected to several reversals of field while passing by the head. The frequency of the erasing field is sufficiently high to produce the required number of reversals. The strength of the field, as seen by the tape, decays as the tape moves away from the head. These conventional methods of erasing magnetic tape may require specially designed degaussing chambers or recording heads.